Ultrafast photodriven intramolecular electron transfer from a zinc porphyrin to a readily reduced diiron hydrogenase model complex

Article abstract of DOI:10.1021/ja100016v

Diiron complexes modeled on the active site of the [FeFe] hydrogenases having the general formula [Fe₂(μ-R)(CO)_6-n(L) _n], where commonly R = alkyl or aryl dithiolate and L = CO, CN ^-, or PR₃, are a promising class of catalysts for use in photodriven H₂ production. However, many of these catalysts are difficult to photoreduce using chromophores that absorb visible light. Here we report the synthesis and spectroscopic characterization of a naphthalene-4,5-dicarboximide-1,8-dithiolate diiron complex [NMI-Fe ₂S₂(CO)₆, 1] and a covalently linked, fixed-distance zinc 5,10,15-tri-n-pentyl-20-phenylporphyrin-NMI-Fe ₂S₂(CO)₆ donor-acceptor dyad (2). The electron-withdrawing nature of the NMI group makes the diiron complex among the most easily reduced hydrogenase mimics reported to date (-0.74 V vs SCE). In the presence of triflic acid, the cyclic voltammogram of 1 showed an increase in current at the first reduction wave at -0.78 V and a new reduction wave at -1.4 V. As the acid concentration was increased, the current at -0.78 V remained constant while the current at -1.4 V increased significantly, which is consistent with a catalytic proton reduction process. Selective photoexcitation of the Zn porphyrin in 2 with 553 nm, 110 fs laser pulses in both toluene and CH₂Cl₂ yielded transient absorption spectra showing a distinct peak at 616 nm, which has been assigned to [NMI-Fe₂S ₂(CO)₆]^-· on the basis of spectroelectrochemical measurements on 1. The 616 nm peak was used to monitor the charge separation (CS) and charge recombination (CR) dynamics of 2, which yielded τ_CS = 12 ± 1 ps and τ_CR = 3.0 ± 0.2 ns in toluene and τ_CS = 24 ± 1 ps and τ_CR = 57 ± 1 ps in CH₂Cl₂. Photoexcitation of the disulfide precursor to 2 in both toluene and CH ₂Cl₂ produced only the singlet and triplet excited states of the Zn porphyrin, showing that electron transfer is favorable only when the diiron complex is present. Photoexcitation of 2 in the presence of trifluoroacetic acid was shown to generate H₂.

Full text of DOI:10.1021/ja100016v

Published on Web 06/10/2010
Ultrafast Photodriven Intramolecular Electron Transfer from a Zinc Porphyrin
to a Readily Reduced Diiron Hydrogenase Model Complex
Amanda P. S. Samuel, Dick T. Co, Charlotte L. Stern, and Michael R. Wasielewski*
Department of Chemistry and Argonne-Northwestern Solar Energy Research (ANSER) Center, Northwestern
UniVersity, EVanston, Illinois 60208-3113
Received January 1, 2010; E-mail: m-wasielewski@northwestern.edu
The development of molecular systems that utilize earth-abundant
first-row transition metals for the photogeneration of H2 has received
increased attention in recent years.1–8 In general, such systems are
composed of a proton reduction catalyst, a chromophore that serves
to harvest light and transfer reducing equivalents to the catalyst,
and a sacrificial electron donor and proton source. Diiron complexes
modeled on the active site of the [FeFe] hydrogenases9 having the
general formula [Fe2(µ-R)(CO)6-n(L)n], where commonly R ) alkyl
or aryl dithiolate and L ) CO, CN-, or PR3, are a promising class
of catalysts for use in photodriven H2 production, and this has led
to numerous studies of assemblies consisting of diiron complexes
2+
in combination with photosensitizers such as Ru(bpy)3 and Zn
porphyrins.2,10,11 To date, however, photoinduced electron transfer
(ET) from a chromophore to a diiron hydrogenase model complex
has been observed in only a handful of cases,12–14 and the ultrafast
dynamics of this process remain elusive. In many other instances,
Figure 1. (left) Chemical structure of 1. (center) X-ray structure of 1 with
ET is inferred from the quenching of the chromophore fluorescence
thermal ellipsoids set to 50% probability (see Table S1). (right) Chemical
structure of 2 (R ) n-C5H11).
in the presence of the diiron complex, but this is an unreliable
indicator of ET since a variety of processes can be responsible for
decreases in fluorescence intensity.15 Indeed, one of the main
the imide functionality are twofold: First, it is a strongly electron-
limitations in this field is that photoinduced ET is often thermo-
dynamically unfavorable because of the very negative reduction
potentials (e-1.2 V vs SCE) of most diiron complexes.
The benefits of employing a diiron complex with a mild reduction
potential were highlighted in a recent report by Streich et al.,16 in
which the authors observed high catalytic turnover from solutions
containing Ru(bpy)32+, [Fe2(µ-Cl2bdt)(CO)6] (Cl2bdt )3,6-dichlo-
robenzene-1,2-dithiolate); Ered ) -0.80 V17 vs SCE), and ascorbate
as a proton and electron source. Although this study clearly
demonstrated the utility of diiron complexes in photodriven H2
production, there remains a need for a greater understanding of
the fundamental aspects of the ET process in diiron complex-
containing systems, especially on the subnanosecond time scale.
This requires covalently linked units in which the distance between
the chromophore and the diiron complex as well as their relative
orientation can be controlled.
Recently, naphthalene-1,8-dithiolates were shown to yield diiron
proton-reduction catalysts that are both robust and redox-tunable.18
The reduction potentials of the diiron complexes can be adjusted
through chemical substitution of the naphthalene; for example, the
addition of electron-withdrawing chloro substituents results in a
+80 mV shift in the first reduction potential of the complex.
Additionally, the authors found that the naphthalene scaffold helps
to stabilize the monoanionic species relative to the analogous
propane dithiolate complex [Fe2(µ-S(CH2)3S)(CO)6], as evidenced
by the Ipa/Ipc ratios (∼0.92 vs 0.35 for [Fe2(µ-S(CH2)3S)(CO)6]) in
their cyclic voltammograms.
withdrawing substituent, which should further stabilize the monore-
duced complex and result in less negative reduction potentials,
making photoinduced ET from an organic chromophore (such as a
Zn porphyrin) to the diiron complex thermodynamically favorable.
Second, the imide serves as a convenient site for attachment of
such a chromophore using well-established coupling strategies. Here
we report the synthesis and spectroscopic characterization of the
NMI dithiolate diiron complex NMI-Fe2S2(CO)6 (1) and the Zn
porphyrin-diiron complex donor-acceptor dyad ZnP-NMI-
Fe2S2(CO)6 (2), which was designed to undergo photoinduced
intramolecular ET.
Complex 1 was synthesized by first reacting naphthalene-1,8-
dicarboxyanhydride-4,5-disulfide19 with p-toluidine in refluxing
pyridine in the presence of Zn(OAc)2 (see the Supporting Informa-
tion). The resulting NMI disulfide was reacted directly with
Fe3(CO)12 without initial reduction to the dithiol to give the diiron
complex. Complex 2 was synthesized in an analogous manner by
condensation of naphthalene-1,8-dicarboxyanhydride-4,5-disulfide
with 5-(p-aminophenyl)-10,15,20-tri-n-pentylporphyrin20 and con-
current metalation of the free-base porphyrin using zinc acetate.
The X-ray structure of 1 (Figure 1) shows the characteristic Fe2S2
butterfly core with an Fe-Fe distance of 2.5004(3) Å. The bond
lengths and angles (Table S1 in the Supporting Information) are
consistent with those of naphthalene dithiolate complexes, indicating
that addition of the strongly electron-withdrawing imide does not,
for example, weaken the Fe-S bonds. The IR spectra of 1 and 2
are nearly identical in the carbonyl region, showing bands at 2078,
2043, and 2005 cm-1 (Figures S1 and S2 in the Supporting
Information), which indicates that the porphyrin has no electronic
interaction with the diiron core in the ground state. The carbonyl
On the basis of these findings, we reasoned that naphthalene
monoimide (NMI) dithiolates would be promising ligands for diiron
proton reduction catalysts in photodriven systems. The benefits of
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Figure 3. Transient absorption spectra of 2 in toluene (top) and CH2Cl2
(bottom) following excitation with 553 nm 110 fs laser pulses. The insets
show the transient absorption kinetics at 616 and 470/469 nm. The spectral
features monitored at these wavelengths correspond to the absorption
features of ZnP+•-[NMI-Fe2S2(CO)6]-• and 1*ZnP, respectively. Nonlinear
least-squares fits to the data are also shown.
V remained constant while the current at -1.4 V increased,
which is consistent with catalytic proton reduction.22
Figure 2. (a, b) Cyclic voltammograms in MeCN (0.1 M Bu4NPF6) at 0.1
V/s scan rate: (a) 1 (1 mM); (b) 1 (1 mM) with triflic acid (5 and 10 mM)
and triflic acid alone (5 and 10 mM). (c) Absorption spectra of electro-
chemically generated [NMI-Fe2S2(CO)6]-• (0 to -0.9 V).
Spectroelectrochemistry was used to obtain the UV-vis spectrum
of 1-• (Figure 2c), which was needed to identify the charge-
separated species in the transient absorption measurements. This
reduced species displays a prominent feature near 615 nm, which
contrasts with the 580 and 700 nm peaks characteristic of [Fe2(µ-
S(CH2)3S)(CO)6]-•.23 The extinction coefficient of 1-• at 605 nm
(ε ) 13 000 M-1 cm-1) was determined by titration of 1 with
cobaltocene (Figure S4), which yielded the same spectrum as shown
in Figure 2a. It should be noted that the absorption spectrum of
1-• is also significantly different from that of NMI-•,24 indicating
that the electron density is localized at the Fe2S2(CO)6 core of 1
and not on the NMI moiety. This is consistent with the observation
that 1 is reduced at -0.74 V versus SCE while NMI itself is reduced
at -1.37 V versus SCE24 as well as with the results of semiem-
pirical unrestricted PM3 calculations on 1-• (Figure S5).
The free energy for photoinduced charge separation (CS) in the
Zn porphyrin-diiron complex dyad 2 was estimated to be ∆GCS
) -0.13 eV in toluene and -0.63 eV in CH2Cl2 (see the Supporting
Information). These estimates were based on the first reduction
potential of 1, the reversible oxidation potential (E1/2 ) 0.62 V),
and the excited-state energy (S1 ) 2.06 eV) of Zn 5,10,15-tri-n-
pentyl-20-phenylporphyrin (ZnP), with corrections for the ion-pair
solvation and Coulombic energies. ZnP within 2 was selectively
photoexcited at its 553 nm Q band (Figure S6) with 110 fs laser
pulses, as 1 has negligible absorbance at that wavelength (Figure
S7). Figure 3 shows the transient absorption spectra of 2 in toluene
stretching frequencies of 1 and 2 are hypsochromically shifted by
∼4 cm-1 relative to those of the unsubstituted naphthalene dithiolate
complex (here designated as complex 3),18 reflecting the decreased
electron density at the Fe(I) centers caused by the imide.
To investigate the influence of the imide functionality on the
redox properties of the diiron complex, cyclic voltammetry was
performed. Complex 1 showed two quasi-reversible waves at
E1/2 ) -0.74 and -1.1 V (all potentials are reported vs SCE)
(Figure 2a) that can be assigned to the [Fe(I)Fe(I)]/[Fe(I)Fe(0)]
and [Fe(I)Fe(0)]/[Fe(0)Fe(0)] couples, respectively. First, it
should be noted that, as expected, the presence of the imide
resulted in reduction potentials that are significantly less negative
(350-400 mV) than those of 3. Furthermore, these values are
among the most positive reported to date for a diiron hydrogenase
model complex.21 Second, the large Ipa/Ipc ratios for the first
and second reductions (0.9 and 0.7, respectively) highlight the
chemical stability of both the mono- and direduced species. In
the presence of 5 equiv of triflic acid, the cyclic voltammogram
of 1 showed an increase in current at the first reduction wave at
-0.78 V and a new reduction wave at -1.4 V (Figure 2b). The
first reduction wave remained irreversible in the presence of
triflic acid when the scan was reversed at -0.9 V (Figure S3).
When the acid concentration was increased, the current at -0.78
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and CH2Cl2 at various time delays, and the corresponding transient
absorption kinetics are shown in the insets. At times <1 ps after
photoexcitation, the transient spectra are characteristic of 1*ZnP,
with a strong absorption at 470 nm and broad features from
550-800 nm superimposed on the bleach of the ground-state
absorption along with a stimulated emission feature at 655 nm. The
spectra evolve in time to develop a distinct peak at 616 nm, which
is assigned to [NMI-Fe2S2(CO)6]-• on the basis of the data shown
in Figure 2c and Figure S4. The broad features due to ZnP+• are
characteristically indistinct,25 so the 616 nm peak was used to
monitor the CS and charge recombination (CR) dynamics of 2,
which yielded τCS ) 12 ( 1 ps and τCR ) 3.0 ( 0.2 ns in toluene
and τCS ) 24 ( 1 ps and τCR ) 57 ( 1 ps in CH2Cl2. These time
constants are consistent with the expectations of Marcus ET theory,
and similar solvent dependencies have been observed for other
donor-acceptor systems.20 Photoexcitation of the disulfide precur-
sor to 2 (ZnP-NMIS2; see the Supporting Information for the
chemical structure) in both toluene and CH2Cl2 produced only 1*ZnP
and 3*ZnP (Figures S8 and S9), showing that ET is favorable only
when the diiron complex is present. Photoinduced ET in our dyad
system was therefore unequivocally identified by the observation
of ZnP+•-[NMI-Fe2S2(CO)6]-• after the initial formation of 1*ZnP.
The observed ∆A values and kinetics showed that the quantum yield
for the formation of ZnP+•-[NMI-Fe2S2(CO)6]-• is approximately
unity in both toluene and CH2Cl2 (see the Supporting Information).
In conclusion, the NMI dithiolate scaffold is a potentially
important building block in the development of covalent arrays for
the study of photodriven ET in diiron complex-containing systems.
We are currently investigating ways to improve the photocatalytic
performance by, for example, incorporating sacrificial donors that
are able to reduce relatively stable cation radicals in acidic media
and extending the lifetime of the charge-separated state.
Acknowledgment. We thank M. T. Vagnini for nanosecond
spectroscopic measurements and Profs. T. Rauchfuss (U. of Illinois
Urbana-Champaign) and R. Eisenberg (U. of Rochester) for helpful
discussions. This work was supported as part of the ANSER Center,
an Energy Frontier Research Center funded by the U.S. Department
of Energy, Office of Science, Office of Basic Energy Sciences,
under Award DE-SC0001059.
Supporting Information Available: Experimental details, including
the synthesis and characterization of 1 and 2; transient absorption
spectra; structural data; H2 formation; and a CIF for 1. This material is
available free of charge via the Internet at http://pubs.acs.org.
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